How to generate a secure random alphanumeric string in Java efficiently?

Question

How do you generate a secure random (or pseudo-random) alphanumeric string in Java efficiently?

Answer 1

http://download.oracle.com/javase/6/docs/api/java/security/SecureRandom.html

From the Javadoc:

SecureRandom random = new SecureRandom();
byte bytes[] = new byte[20];
random.nextBytes(bytes);